Structure of DHA and EPA

Chemical structure of docosahexaenoic acid, or DHA (22:6n-3), and eicosapentaenoic acid, or EPA (20:5n-3).

Reference: Lansbury Research Site

Polyunsaturated fatty acids (PUFAs) can be divided into two subcategories, the omega-3 and the omega-6 fatty acids. Both the omega-3 and the omega-6 fatty acids are considered "essential" because they cannot be synthesized by humans and thus must be obtained through diet or supplementation. Alpha-linolenic acid (ALA), classified as an omega-3 fatty acid, is found in certain plant oils, seeds, green leafy vegetables, beans, and nuts. Linoleic acid, classified as an omega-6 fatty acid, is found in grains, meats, and the seeds of most plants.

Through an enzymatic process of desaturation, ALA produces EPA and DHA. EPA and DHA are precursors to a group of eicosanoids (prostaglandins, thromboxanes, and leukotrienes) that have anti-inflammatory, antithrombotic, antiarrhythmic, and vasodilatory properties.[2] Arachidonic acid is a derivative of linoleic acid and a precursor to another group of eicosanoids responsible for proinflammatory and prothrombic effects. ALA and linoleic acid use and compete for the same enzymes in the production of EPA and arachidonic acid. The ingestion of fish and fish oils provides both EPA and DHA directly, thereby avoiding the competition for enzymes for the conversion of ALA to EPA. Read more

Reference: Flaxcouncil.ca